The invention relates to improvements in paper machine dryer drums, and more particularly to an improved condensate removal system with a siphon tip capable of removing a thin layer of rimming condensate and condensate which gathers in a puddle.
In a papermaking machine after the web is dewatered and pressed, it is passed to the dryer section which conventionally embodies a plurality of cast iron steam heated rotating dryer cylinders. Steam enters each of these dryers and condenses as it transfers heat to the dryer shell, and the dryer shell in turn transfers heat to the paper causing the moisture in the paper to evaporate. It is essential to producing a satisfactory paper sheet that the dryer drum transfer heat efficiently and uniformly to the sheet, and it is essential to satisfactory and economical operation that effective and efficient heat transfer occur from the steam in the drum to the shell. A large factor in controlling and attaining effective heat transfer is the removal of the condensate within the drum. This condensate may be ponding, cascading or rimming, and the state of the condensate depends upon a number of factors, but the siphon assembly which is provided to remove the condensate must be capable of satisfactory removal whether the condensate is ponding, cascading or rimming.
When the condensate is rimming, the heat must be transferred through it, and this is a thermal problem. The thicker the condensate layer, the higher the thermal resistance. When the condensate is not rimming, the problem becomes a mechanical one, and if the condensate layer becomes too large, it takes longer to speed up, and the drive loads increase and the drive torque becomes erratic which has a deleterious effect on the quality of the paper being manufactured. Therefore, it is important to minimize the amount of condensate in the dryer drum at all times.
It is, therefore, an important object of the present invention to provide an improved dryer drum and siphon condensate removal system which minimizes the amount of condensate in the drum at all times and is capable of handling the condensate whether it be ponding, cascading or rimming.
In general in present commercial dryers, the steam enters the dryer drum often through a backside journal, and the condensate is evacuated through a frontside journal along with some uncondensed steam which is termed blow-through. The components of the siphon assembly are the siphon shoe or tip which is usually positioned adjacent the inner surface of the shell, the radial pipe through which the condensate flows after it is picked up by the shoe, the siphon elbow which turns the flow of condensate to a horizontal direction, the horizontal pipe, and the rotary steam fit. These items make up the dryer siphon assembly and are each important, but a salient feature of the instant invention is the design of the shoe or tip which is capable of effective removal of the condensate and a thorough and a more complete removal of condensate for the conservation of energy and the production of a better paper web.
The achieving of a structure which can operate with a thinner layer of condensate will reduce the insulating effect of the condensate. It has been found that the condensate depth decreases consistently with decreasing clearance between a siphon shoe or tip to a point of approximately 0.060". Once that point has been reached, the condensate cannot be reduced in thickness by any further reduction in siphon clearance, and this feature is taken into consideration in connection with the design of the siphon tip of the instant invention.
As stated above, the condensate may take different forms in either cascading, puddling or rimming, and it is well known as recited in White, R. E. "Residual Condensate, Condensate Behavior, and Siphoning in Paper Driers", TAPPI, Vol. 39, No. 4, Apr. 1956, p. 228-233, that at lower dryer speeds, condensate which is still in the dryer drum will form a puddle in the bottom of the dryer. At slightly higher speeds, this condensate starts to climb the wall of the dryer shell and then cascade back into the puddle. An excessive amount of condensate cascading off the dryer wall will produce a large and erratic drive load. At higher speeds, the condensate forms a thin circumferential layer on the inside dryer surface, and this layer acts as a thermal insulator. Under these rimming conditions, an excessive amount of condensate in the dryer will produce an excessive thermal resistance. To avoid both of these operating conditions, the amount of condensate in the dryer must be kept at a minimum, and in accordance with the features of the present invention, the design of the siphon tip greatly influences the value of this minimum. During nonrimming conditions, the siphon tip will evacuate condensate only when it is submerged in the condensate puddle, that is, when the siphon tip is in the region where it projects downwardly so as to be in the puddle. During the rest of the cycle, the siphon tip is exposed only to the steam which is under pressure in the drum and hence only blow-through steam is being evacuated, rather than condensate.
In order to evacuate as much condensate as possible during the time the siphon tip is submerged, conventionally siphon tips have heretofore been designed for nonrimming conditions and have a large spacing between the siphon tip and the dryer shell, or a large opening in the bottom of the siphon tip. With this construction and the use of a large opening, the blow-through flow rate of steam is excessive when the siphon tip is not submerged. Further, the amount of condensate in the dryer is very large during high speed rimming operation.
In accordance with the features of the present invention, the foregoing problems are minimized by the design of the siphon tip such that a relatively small spacing, optimally less than 0.08" is provided between the siphon tip and the inner surface of the dryer shell. It has been discussed that the spacing has an effect on the thickness of the layer of rimming condensate by Calkins, D. L., "The Effects of Siphon Clearance on Dryer Performance", The Johnson Corporation, Three Rivers, Michigan, 1966. The small spacing in conventional designs, however, has a disadvantage of providing a very poor nonrimming performance. It has been found that in order to evacuate condensate at a rate of 3,250 lb/hr, and keep the puddle depth less than 8", the pressure differential across the siphon assembly must be in excess of 7.0 psi. This produces a blow-through flow rate in excess of 450 lb/hr.
A further feature of the invention substantially improves the condensate removal performance in that the profile of the edge of the siphon tip which first contacts the condensate puddle is uniquely constructed. This edge slopes in an angular face toward the inner surface of the shell and has a large projected opening which is tapered down to a small siphon tip to dryer shell spacing. The angle of the ram face which faces in the direction of rotation of the dryer should be in the range of 10.degree. to 45.degree. and of a height which is preferably in the range of 0.2" to 1.0". This leading edge acts as a channel which forces the condensate into the small opening and has a ram effect greatly enhancing the nonrimming performance of the siphon which is mounted very close to the dryer shell. It has been found that this type of siphon can evacuate condensate at a rate of 3,250 lb/hr and keep the puddle depth to less than 4" with a pressure differential across the siphon assembly at only 6.5 psi which produces a blow-through flow rate of only 400 lb/hr.
A further feature of the invention is to provide a siphon tip with a center enlarged chamber portion. This chamber has an axially extending partition dividing the chamber into trailing and leading edge chamber portions. The partition provides a better surface on which to rest the siphon tip and also provides a wall to keep the condensate which enters the siphon tip from the leading side from flowing through the siphon tip and out the trailing edge.
A further object of the invention is to provide an improved siphon tip of a design which is structually efficient to manufacture and which is capable of the improved removal of condensate from a steam dryer drum.
Other objects, advantages and features will become more apparent with the teaching of the principles and concepts of the invention in connection with the description of the preferred embodiment in the specification, claims and drawings, in which: